Control of overload situations in frame relay network which discards the contents of a virtual-channel-specific buffer when said buffer is full

ABSTRACT

The invention relates to a method for congestion management in an FR network. The method comprises determining the virtual channel associated with a frame to be transmitted when it is received from a subscriber connection at a subscriber node. In order for the load in the network to be reduced, (a) data is buffered at the input boundary of the subscriber node to virtual-channel-specific buffers (62 1  . . . 62 n ), (b) the fill rate of said buffers is monitored continuously, and (c) when a frame is received in a virtual-channel-specific buffer which is full, substantially all the contents of the buffer are discarded.

This application claims benefit of international applicationPCT/FI94/00534, filed Nov. 29, 1994.

The invention relates to a method according to the preamble of appendedclaim 1 for congestion management in a frame relay network, and to asubscriber node according to the preamble of appended claim 3.

Congestion means a situation in which the number of transmissionrequests exceeds the transmission capacity at a certain network point(called a bottle-neck resource) at a specific time. Congestion usuallyresults in overload conditions, as a result of which the buffersoverflow, for instance, and so packets will be retransmitted either bythe network or the subscriber. The function of congestion management(CM) is to maintain a balance between the transmission requests and thetransmission capacity so that the bottle-neck resources operate on anoptimal level, and the subscribers are offered service in a way thatassures fairness.

Congestion management can be divided into congestion avoidance (CA) andcongestion recovery (CR). Congestion avoidance methods aim at preventingthe occurrence of congestion in the network by dynamically adjusting thebandwidth of the subscribers in accordance with network congestionconditions and/or by altering the network routing so that part of thetraffic load of the bottle-neck resources is shifted to idle resources.The purpose of recovery methods, in turn, is to restore the operation ofthe bottle-neck resources to an optimal level if the avoidance methodshave failed to prevent the occurrence of congestion.

The frame relay (FR) technique is a packet-switched network techniqueused for the transmission of frames of varying length in place of thepacket-switched network connections presently in use. The protocol(X.25) applied generally in the present packet-switched networksrequires plenty of processing and the transmission equipment isexpensive, as a result of which the speeds also remain low. Thesematters are due to the fact that the X.25 standard was developed whenthe transmission connections used were still rather prone totransmission errors. The starting point of the frame relay technique wasa considerably lower transmission line error probability. It hastherefore been possible to discard a number of unnecessary functions inthe frame relay technique, which makes the delivery of frames rapid andefficient. The Frame Mode Bearer Service is described generally in CCITTrecommendation I.233 (Reference 1) and the associated protocol inrecommendation Q.922 (Reference 2). Congestion in an FR network andcongestion management mechanisms are described in CCITT recommendationI.370 (Reference 3). For a more detailed description of the FRtechnique, reference is made to An Overview of Frame Relay Technology,Datapro Management of Data Communications, McGraw-Hill Incorporated,April 1991, (Reference 4) as well as the above-mentionedrecommendations.

As a last resort in cases of extremely severe congestion in a framerelay network, frames can be discarded. Situations of this kind arisewhen both the transmission capacity of the network and the bufferingcapacity of a single node are exceeded. In the congestion managementmechanisms presently used, frames are discarded almost randomly both onthe periphery of the network (at subscriber nodes) and in the middle ofthe network (at trunk nodes). A frame is discarded if it cannot bebuffered due to lack of space, or if the fill rate of the buffer issufficiently high and the frame contains an indication which allows itto be discarded in the event of congestion, for instance. Nothing isdone to affect the buffer itself; it continues to be emptied to the datalink as normal. If the frame is not discarded until at a congested node,the other resources of the network are loaded in vain. The frame must beswitched through the network up to the point where it is discarded, i.e.to the congested node, which means wasting the resources.

The object of the present invention is to obviate the drawback describedabove and to provide a new type of congestion management method for aframe relay network, said method allowing unnecessary loading of thenetwork to be avoided. This is achieved with a method according to theinvention, which is characterized by what is disclosed in thecharacterizing part of appended claim 1. The FR network subscriber nodeaccording to the invention is characterized by what is disclosed in thecharacterizing part of appended claim 3.

The idea of the invention is to empty the contents of an entirevirtual-channel-specific buffer at a time on the periphery of thenetwork at a subscriber node where the frames are buffered tovirtual-channel-specific buffers. (The virtual channel refers to avirtual connection portion having the length of one transmission linkwhile the virtual connection is the actual packet-switched end-to-end FRconnection.)

The method according to the invention makes it possible to avoiddiscarding random frames, on account of which the applications do nothave to retransmit packet sequences as often as before.

In the following, the invention and its preferred embodiments will bedescribed in greater detail with reference to the examples illustratedin the accompanying drawings, in which

FIG. 1 illustrates a typical operating environment of the methodaccording to the invention,

FIG. 2 illustrates an FR network subscriber node according to theinvention,

FIG. 3 illustrates the format of a frame to be delivered in an FRnetwork, and

FIG. 4 illustrates a trunk node of an FR network employing the methodaccording to the invention.

A frame relay network can be used by several different applications,which do not require similar services. It is therefore advantageous toemploy the method of the invention in a network where the services aredivided into different classes according to the applications, takinginto account the two most significant parameters (frame loss probabilityand delay). Such a solution is disclosed in Finnish Patent ApplicationNo. 925671. In this application, it is suggested that the services bedivided into three classes as follows:

the first service class (class 1) offers interactive service where thedelay is kept short,

the second service class (class 2) offers a low frame loss probabilitywithout the delay having any great significance, and

the third service class (class 3) offers both a short delay and a lowframe loss probability.

Each trunk connection of a network realized in this way hasservice-class-specific buffers, one for each service class. A subscribernode, in turn, has virtual-channel-specific buffers on the side of thesubscriber interface. These solutions will be described more closelybelow; otherwise reference is made to the Finnish patent applicationcited above.

(Subscriber) applications using a frame relay network send data towardsthe network in frames, typically a so-called window at a time. If theapplication and the protocol used by it detect that a frame included ina window has been lost, the entire window is usually retransmittedinstead of transmitting only the frame that has been lost. Thus the lossof one frame in the network affects the application in the same way asthe loss of an entire window. In each case, the application protocolmust retransmit a whole windowful of frames.

FIG. 1 shows an FR network offering public network services, that is, aframe relay network 12 interconnecting local area networks 11 ofdifferent offices A . . . E of a single corporation or a plurality ofcorporations. The local area network 11 of each office has access to theFR service via a local area network bridge 13 and a data link indicatedwith the references 14a . . . 14e, respectively. The connection betweenan FR subscriber A . . . E and an FR network node N is known per se,wherefore it will not be described more closely herein. More detailedinformation about local area networks and bridges used in theirinterconnection can be found e.g. in an article by Michael Grimshaw LANInterconnections Technology, Telecommunications, February 1991, and inLahiverkko-opas, Leena Jaakonmaki, Suomen ATK-kustannus Oy, 1991, whichare incorporated herein by reference.

A typical feature of the known node structure of the FR network is thatthe same buffer is used for all frames, assuming that they are routed tothe same physical connection. According to the present invention, on thecontrary, buffers corresponding to the above-described service classesare preferably provided at the output boundary of all network nodes andat the input boundary having trunk connections. FIG. 2 illustrates thiskind of solution at a trunk node in the network. The node receives FRframes originally assembled in the bridges 13 of the subscriberconnections (FIG. 1). The frame of the subscriber LAN 11 is insertedinto the information field of the FR frame in the bridge 13 (with theexception of timing bits and other similar bits). FIG. 3 illustrates theinsertion of a LAN frame 38 into the information field of an FR frame39. It also shows a typical FR network frame format where the addressfield preceding the information field comprises two octets (bits 1 to8). The bits 3 to 8 of the first octet and the bits 5 to 8 of the secondoctet form a data link connection identifier DLCI, which indicates tothe node e.g. the virtual connection and virtual channel to which aparticular frame belongs. The virtual channels are distinguished fromeach other by means of the data link connection identifier. The datalink connection identifier, however, is unambiguous only over a singlevirtual channel, and it may change in the node on transition to the nextvirtual channel. The bit 2 of the second address field octet, called aDE bit (Discard Eligibility Indicator), is also significant for thediscarding of frames. In accordance with the CCITT recommendation, it isallowable to discard a frame e.g. under congestion conditions if the DEbit of the frame has been set to one. As the other bits in the FR frameare not relevant to the present invention, they will not be describedmore closely herein. References 2 and 4 mentioned above are referred tofor a more detailed description.

At a subscriber node on the periphery of the network (FIG. 2), thesubscriber connections 14a, 14b, etc., (which in the example illustratedin FIG. 2 are connected to the same node) are connected at first to anidentification and control unit 61, which receives FR frames formed inbridges 13 (FIG. 1). The identification and control unit 61 reads thedata link connection identifier DLCI from the address field of theframe, and forwards the frame to an input buffer 62₁ . . . 62_(n)corresponding to the virtual connection indicated by the identifier.Each data link has a specific selector S3 which selects frames from theinput buffers of each virtual channel and forwards them further to acentralized router 16, which routes the frames further to aclassification unit 43 of the correct data link (the figure shows onlyone outbound data link). The classification unit 43 reads the identifierDLCI from the address field of the frame and selects from table T theservice class corresponding to the virtual channel indicated by theidentifier. On the basis of the classification it has completed, theclassification unit 43 applies each frame to an output buffer 64a, 64bor 64c corresponding to the service class of the frame. Each outbounddata link thus has three output buffers, one for each service class. Theselector S2 selects the frames from the service-class-specific outputbuffers 64a . . . 64c and forwards them to the trunk connection.

Traffic transmitted by the subscribers over the FR network is thusbuffered on the input side of the subscriber node specifically for eachvirtual connection. Incoming frames 39 are chained dynamically over eachvirtual connection. Depending on the service class of the virtualconnection, the chain length has a predetermined allowable maximumvalue; this is smaller in service classes 1 and 3 and greater in serviceclass 2. The selector S3 reads the buffers 62₁ . . . 62_(n) e.g. inproportion to the amount of traffic allocated to them, whereby thefairness principle is met.

When a node in the network is congested, the aim is to reduce traffic onthe virtual connections running through it already at the subscribernode at the source end of the network; thus the frames will not load theother resources of the network only to be discarded on reaching thecongested node. According to the invention, this is carried out in sucha way that the identification and control unit 61 of the subscriber nodemonitors the fill rate of each virtual-channel-specific buffer 62₁ . . .62_(n) (in a manner known per se), and discards all the contents of abuffer if the buffer receives a frame while being full. At the sametime, the identification and control unit discards this incoming frame(which may be, for instance, the first frame of a longer frame packet).The frames received thereafter are stored in the emptied buffer. In FIG.2, the operations of the identification and control unit for controllingand emptying the buffer are indicated with bidirectional arrows FC.

Discarding all the contents of the buffer at a time relieves congestionin the network considerably more than discarding only a few frames. Thelevel of congestion in the network becomes considerably lower when theentire virtual-channel-specific buffer is emptied, the length of thebuffer being typically dozens of frames. Emptying the buffer ensuresthat the virtual channel in question hardly loads the network at all fora while. This also makes it possible to avoid unnecessary loading of thenetwork, as frames which would have to be discarded at a later stagebecause of congestion are not supplied to the network.

In view of the applications using the network, the method of theinvention has the advantage that the frame loss probability is lowerthan before, as a result of which the load caused by retransmissions isalso reduced. Discarding a whole buffer leads very close to a situationwhere an entire window of frames is discarded and must be retransmittedby the application protocol. It is therefore advantageous to adjust thelength of a virtual-channel-specific buffer to correspond substantiallyto the length of a window of frames sent at a time by an applicationusing the network.

FIG. 4 illustrates the processing of frames at a trunk node of thenetwork. An FR frame 39 of the format described above is received atfirst by a classification unit 43 specific for each data link. Theclassification unit 43 reads the data link connection identifier fromthe address field of the frame, and selects the service classcorresponding to the virtual channel indicated by the identifier. Thevirtual channels and the respective service classes are stored in atable T. On the basis of the classification it has completed, theclassification unit 43 applies each frame to an input buffer 44a, 44b or44c corresponding to the service class of the frame. Each inbound datalink has thus three input buffers, one for each service class. Each datalink has a specific selector S1 which selects the frames from theservice-class-specific buffers and forwards them within the node. On theoutput side of the trunk node, the frames are connected to an interfacecorresponding to their particular data link. At this interface they aresupplied in accordance with the service class selected on the input sideof the node to one of the three service-class-specific output buffers45a, 45b or 45c, from which the selector S2 reads the frames further tothe trunk connection. Each outbound data link has thus three outputbuffers, one for each service class. Alternatively, classification unitsmay be provided separately for each data link even on the output side ofthe node, in which case classification data need not be transferredwithin the node.

Although the invention has been described above with reference to theexamples shown in the accompanying drawings, it will be obvious that theinvention is not restricted to these examples, but can be modifiedwithin the scope of the inventive concept disclosed above and in theappended claims. In its details, the structure of a subscriber node, forexample, may vary in many ways even though frames are discardedaccording to the principle of the invention. There may be, for instance,a separate fill rate control unit per each virtual-channel-specificbuffer, said control unit transmitting data on the fill rate to a unit61, which (in addition to identifying the virtual channel) merelyempties the buffer.

We claim:
 1. A method for congestion management in an frame relaynetwork comprising subscriber nodes to which subscribers (A . . . E) areconnected over data links (14a . . . 14e), said method comprisingdetermining the virtual channel associated with a frame (39) to betransmitted when it is received from a subscriber connection at asubscriber node, characterized in thatdata is buffered at the inputboundary of a subscriber node to virtual-channel-specific buffers (62₁ .. . 62_(n)) . the fill rate of said buffers is monitored continuously,and when a frame is received in a virtual-channel-specific buffer whichis full, substantially all the contents of the buffer are discarded. 2.A method according to claim 1, characterized in that the length of thevirtual-channel-specific buffer (62₁ . . . 62_(n)) is adjusted tocorrespond substantially to the length of the frame packet sent at atime by an application using the network.
 3. A subscriber node of aframe relay network, to which node subscribers (A . . . E) of thenetwork are connected over data links (14a . . . 14e), said nodecomprising input buffers at the input boundary of the node, and outputbuffers at the output boundary of the node, and means (16) for relayingframes from a input buffer to a desired output buffer, characterized inthat it comprisesvirtual-channel-specific buffers (62₁ . . . 62_(n)) atthe input boundary, first means (61) for monitoring the fill rate ofsaid virtual-channel-specific buffers, and second means (61) responsiveto said first means (61) and to the frame received for discarding thecontents of a virtual-channel-specific buffer.
 4. A subscriber nodeaccording to claim 1, characterized in that said first and second meansare provided at the same identification and control unit (61), throughwhich the frames received at the subscriber node are supplied to thevirtual-channel-specific buffers (62₁ . . . 62_(n)).